Reports quoting witnesses and human rights activists say Syrian security forces arrested hundreds of people today in raids across Syria -- part of an escalating government crackdown aimed at stamping out growing nationwide protests.

President Bashar al-Assad has dispatched army troops and tanks to put down the eight-week uprising against his family's 40-year rule.

Rights activists say gunfire was heard in a western suburb of Damascus today as hundreds of people were being arrested in the capital.

An activist in Damascus told the BBC that at least three civilians had been killed in the city today.

Rami Abdul-Rahman, director of the Syrian Observatory for Human Rights, said more than 250 people also were arrested in Banias today after troops and tanks were deployed in the oil refinery city on May 7.

A United Nations spokesman, meanwhile, said today that Syrian authorities have stopped a UN humanitarian team from visiting the city of Daraa where hundreds are said to have been killed.

Spokesman Farhan Haq said the UN is "trying to clarify why it hasn't had access."

According to human rights groups, more than 600 people have been killed and 8,000 have been jailed or disappeared across Syria in the eight-week-old antiregime campaign.